In modern communication systems, timing recovery at a receiver is an essential issue for successful communication. Usually, at a transmitter, digital values sampled with a clock frequency are processed and transmitted, e.g. via an optical signal to a remote receiver. During transmission, the optical signal is impaired by polarization effects, dispersion, attenuation, noise and other effects. At the receiver, the clock frequency forming a time basis need to be recovered in order to capture the transmitted samples without introducing any further errors due to clock frequency mismatch. Time recovery errors may, in particular in optical transmission systems, further be introduced due to optical polarization effects like differential group delay (DGD), which may negatively affect the timing recovery accuracy. In particular, half-Baud DGD or multiples thereof in combination with certain states of polarization (SOP) may impose critical conditions for timing recovery. The timing recovery should be also robust to all optical and electrical distortions.
In order to recover the time basis, square-timing recovery (STR) may be performed in time domain. Essentially, the STR approach applies a nonlinear operation, e.g. the square of the signal, to obtain nonlinear mixing products, i.e. the clock tone. Prior to STR, a time domain filter which is a FIR filter (FIR: Finite Impulse Response) for enhancing spectral components around the clock frequency, i.e. the clock tone, may be applied. The filtered signal is then oversampled and squared to obtain the clock tone.